Code 8Z: Drive Train Error Detected: While traveling in a straight line at 2.5 mph (4.0 km/h) or greater with a greater than 50% throttle request, high current is detected in one traction motor and not the other.

Jack up both traction tires and request travel. If only one traction tire turns while both traction motors are turning, repair the drive train as necessary.

I think it also may be caused by a bad encoder or maybe brake its being applied.